In the oil and gas industries, petrochemicals and hydrocarbon gases are extracted from deep in the earth through pressure bearing tubulars or “tubing”. The tubing forms a conduit from the rock where the petrochemicals reside to the surface where it is terminated at the Wellhead or Christmas Tree. The wellhead is equipped with a number of valves to control and contain the pressure which is present in the tubing.
The oil or gas flows from source rock which may exist in a layer of just a few feet to many hundreds of feet. The quality and productivity of the rock may vary over distance, and water or other undesirable elements may exist at certain points. Usually it is best practice to produce over the entire oil bearing interval and for any water to be produced along with the oil. Towards the latter stages of a well's life, the water production will generally increase at the expense of oil production. Production optimisation will depend on minimising the water production which will maximise the oil production.
Production may also be lost to “thief” zones. Thief zones are areas of rock penetrated by the wellbore which have less pressure than others. Crossflow can occur from a good high pressure zone to a poor low pressure zone. (See FIG. 1) Obviously, this is inefficient. Production optimisation will depend on isolating the thief zone until such time as the good high pressure zone has depleted to the extent that the pressure is the same or lower than the thief zone. Once the isolation has been removed, both zones may be allowed to flow to surface.
The production may initially be optimized by “shutting off” thief zones or water producing zones. Firstly, these zones must be identified and targeted. Instruments lowered into the wellbore on a wireline cable allow pressure, temperature, flow measurement and flow composition readings to be taken. Following analysis, a second intervention into the well may be conducted to mechanically close off the undesirable zone(s). A variety of equipment is available for this but most will dictate permanently closing off a part of the wellbore, which action may be undesirable in later years.
A technology whereby the zones of a well may be individually opened or closed to help optimise the production from that well is called “smart well” technology. Differing zones are mechanically separated and isolated by packer assemblies (See FIG. 2 demonstrating a well with three of these devices). Flow from the zones is received through a valve which may allow on/off or incremental flow. Most of these valves feature a sleeve which uncovers flow ports in the outside diameter of the tool. Many of these valves may be installed in a well with surface control being provided by means of electric cables, hydraulic control lines or other means. Most smart well systems require a physical link from the bottom of the well or the valve apparatus to surface in order to provide hydraulic contact, electrical contact or both. Not only is this expensive, it becomes a source of unreliability. Failure of one part of this type of system may compromise all of the system. Obviously, the complexity (and unreliability) of the installation increases proportionally with the number of valves and the increase in control lines and/or electric lines, splices and connections.
Equipment which uses this type of physical link must be installed when the well is new. It is not capable of retrofitting into an existing well.
The ability to repeatedly open and close various zones from surface allows true optimisation without the need to intervene in the well for data collection or for installation of shut off equipment. Also, isolated zones may easily and quickly be re-opened for evaluation and potential production later in the life of the well or simply just for re-evaluation purposes.
Many wells are not suited to intervention techniques due to the great cost associated with these operations. These may be sub sea wells where no facilities exist to support the intervention, high pressure wells where safety is a prime consideration or remote wells where also, no facilities exist.
Recent innovations in the electro magnetic and acoustic fields have sought to mitigate the disadvantages of the physical link to surface and associated unreliability. Other similar developments include pressure measurement at the proximity of the valve device to detect flow modulation signals. All these devices may offer a greater degree of flexibility and possibly higher reliability. They utilise batteries for powering the signal detection element of their design and accordingly, management of power consumption is of critical importance to guarantee a long service life. Once the actuation signal has been detected, the flow control valve must be operated. This may be performed by using an hydraulic pump providing pressure to a piston arrangement or by using an electric motor acting on a leadscrew. As large forces are required, both these methods consume large amounts of power requiring a substantial battery pack to provide a modest amount of openings and closings of the valve sleeve.
A large number of downhole tools exist which utilise well pressure for their operation. Some contain a Nitrogen chamber which allows internal hydraulics to be referenced to well pressure or to provide a reservoir of trapped energy. An alternative method is to reference an hydraulic piston to an air chamber. This pressure differential between well hydrostatic pressure and atmospheric pressure may provide large shifting forces. The pressure imbalance may be used to generate large forces for opening or closing valves, packing off rubber sealing elements or performing significant mechanical functions.
Multi shot devices do exist performing similar functions but are less common. These tools convey their finite energy from surface by using batteries, explosives or large volume air chambers dictating a limited number of cycles before retrieval for refreshment is required. In the case of air chambers, the number of cycles is dictated by the finite volume of the air chamber. Other devices requiring multiple cycles may have a mechanical link to surface which is utilised to provide electrical or hydraulic energy to the downhole location.
The valve or device which must be operated downhole requires a certain amount of energy to physically change its position. Formation of scale, wax, corrosion or friction may require quite large forces to perform this action.
It may be possible that a pressure differential exists between the production tubing and the casing which might be utilised. This type of differential may be sufficient to provide the large forces required but in order to access this pressure source, modification of the tubing to communicate with the annulus will be required. This is a complex operation and may be undesirable in terms of the safety implications for operation of the well.
Accordingly, the present invention seeks to provide an alternative means of providing multiple cycles of large operating forces available without the need of retrieval to surface for replenishment of the energy source and without the need for communicating with surface by way of an hydraulic conduit or electrical link.